The present invention relates to macroporous asymmetrical hydrophilic membranes made of a synthetic polymer and to a process for their preparation.
Since the introduction of asymmetrical membranes made of cellulose acetate, by Loeb and Sourirajan, (S. Sourirajan, Reverse Osmosis, Logos Press, London 1970), and made of hydrophobic polymers (U.S. Pat. No. 3,615,024), numerous membranes have been developed and proposed, in particular for separating out low molecular weight and macromolecular constituents dissolved in water. The structure and suitability of these membranes have been indicated in the literature (Desalination, 35 (1980), 5-20), and they have also been used with success in industry or for medical purposes.
Many of the membranes described have properties which are particularly advantageous for solving specific tasks. However, a membrane which can be used equally well in different application areas is not available. Owing to their chemical structure and the way in which they are put together, the membranes described can be optimally suitable only for specific separation problems. This results in the fundamental requirement of always developing new membranes for new problems.
Thus, for example, the concentration polarization, which always arises on membrane surfaces and which, in many cases, leads to membrane fouling and to the formation of a secondary membrane, has the effect that the qualitative and quantitative chemical composition of the polymer forming the membrane frequently has a strong influence on the properties of the membrane. This is in addition to the influence of the membrane structure, for example, its asymmetrical structure, or the membrane geometry which is expressed in its capillary or microporous structure.
Relatively hydrophilic ultrafiltration membranes made of cellulose acetate are highly suitable, for example, for separating certain proteins from their aqueous solution, since the membranes in contact with aqueous solutions have only weak adsorptive properties for these proteins. However, these membranes are not sufficiently stable to aggressive chemical agents, in particular those which are capable of effecting hydrolysis of the polymer forming the membrane, and the properties of these membranes also change in an undesirable manner on exposure to temperatures above 40.degree. C. Both the membrane properties mentioned considerably restrict the use of such membranes.
Admittedly, asymmetrical hydrophobic membranes, for example, those based on polysulfone, polyvinylidene fluoride or other hydrophobic polymers, have satisfactory resistance to hydrolysis and to elevated temperatures and to oxidizing agents. However, when in contact with dissolved macromolecules, dispersions and the like, for example, oil emulsions, cataphoretic varnishes or proteins, which tend to form deposits, these membranes frequently and rapidly lose their activity, inter alia due to precipitation and adsorption of constituents of the solution onto and into the membrane.
To avoid these disadvantages it has already been proposed to develop hydrophilized membranes from hydrophobic and stable polymers. For example, the addition of aerosils to polysulfones has enabled their wettability in aqueous solutions to be improved. It has also been proposed to prepare membranes from mixtures of polyvinylidene fluoride and polyvinyl acetate. In order to impart hydrophilic properties to membranes made from this polymeric mixture, it is necessary, however, to subject them to hydrolysis in order to convert the acetate groups contained in this polymeric mixture into OH groups. The attempt to prepare hydrophilic membranes having satisfactory properties by preparing them from a mixture of a hydrophobic polymer and an originally hydrophobic polymer, for example, from polyvinylidene fluoride and polyvinylpyrrolidone, has not had the success desired, because it was only possible to prepare membranes from this mixture which contained at most 15-20% by weight of polyvinylpyrrolidone, but which did not have the properties of products according to the invention.
It has also been proposed to prepare hydrophilic membranes by starting from a solution of a hydrophobic polymer which contains, relative to the total weight of the solution, up to 150% by weight of polyethylene glycol (Polymer-Bulletin 4, 617-622, 1981). However, membranes of this type do not have adequate hydrophilic properties, since the hydrophilic component contained in them is eluted from the coagulated membrane by the aqueous coagulating liquid during the coagulation process.
German Offenlegungsschrift No. 2,651,818 describes a membrane which is comprised of a mixture of polysulfone and sulfonated polysulfone. The known membrane can contain up to 30% by weight, relative to the total weight of the polymeric mixture forming the membrane, of the hydrophilic polymer component. However, the known membrane, when used as an ion exchange membrane, has the fundamental disadvantage that it adsorbs positive counter-ions and repels co-ions of the same charge as the membrane.
It has also been proposed (German Auslegeschrift No. 2,829,630) to prepare a membrane having hydrophilic properties from a hydrophobic polymer by starting from a polysulfone solution which contains low molecular weight salts and preparing membranes from this solution in a manner which is in itself known, using the phase inversion method. However, the water sorption of these known membranes is unsatisfactory, because the salts are removed from the membranes in their preparation or during use, so that the hydrophobic character of the polymer which forms the membrane essentially determines its properties.
A porous membrane which is comprised of a mixture of polyvinylpyrrolidone and aromatic polysulfone is described in J. Appl. Pol. Sci., 21, 1883-1900 (1977). However, this reference does not provide any directive for preparing, for example, membranes which are fouling-resistant, reactive, biocompatible or suitable for hemodiafiltration. The authors admittedly use mixtures containing polyvinylpyrrolidone, but with the object of thereby obtaining high viscosities and good fiber-forming properties. For this reason they only employ polyvinylpyrrolidone which has a maximum molecular weight of 40,000, preferably only 10,000, with the express object of eluting this additive as early as during membrane formation in the aqueous coagulation bath so that no polyvinylpyrrolidone remains in the membrane (J. Appl. Pol. Sci. 20, 2377-2394 (1976)). A membrane which has the specific properties in accordance with the invention can therefore not be produced.
The prior art has sought to provide hydrophilic membranes which have a high water sorption and which possess only to a slight extent the disadvantages summarized under the heading "membrane fouling", but this has still not been satisfactorily accomplished by the state of the art. Another object, not yet achieved by known membranes, is to form asymmetrically and macroporously structured and sufficiently hydrophilic membranes, which ensure high permeability and, at the same time, satisfactory pressure-stability and safe handling. The desirable properties of the type of membrane mentioned also embrace improved resistance to dilute organic solvents, a wider spectrum of the molecular weight exclusion limit (in particular in the transition range from ultrafiltration to microfiltration or to hyperfiltration) and the capability of use in the medical field, for example, for plasmapheresis or hemodiafiltration. In order to be suitable for use in the medical field, the membrane must have considerably higher diffusive permeabilities for toxic metabolites having a molecular size below the particular molecular weight exclusion limit of the membrane and also a good biocompatibility in contact with blood.
Admittedly, there are available hydrophilic membranes which have a high diffusive permeability, for example, gel-like membranes made of regenerated cellulose or of a polycarbonate block polymer, and which have sufficiently high water absorbency, but the known hydrophilic membranes of this type do not have a macroporous and asymmetrical structure, which is the prerequisite for additionally obtaining, for example, high mechanical permeability and pressure stability. Moreover, these hydrophilic membranes again do not have the virtues (for example, the chemical resistance) of hydrophobic membranes.
Admittedly, hydrophobic polymers are capable of forming macroporous asymmetric structures, but usually inadequate wettability and blood compatibility thereof, and also their low diffusive permeabilities preclude, for example, the use in medicine of membranes made of this material.